What could have been the day Rory McIlroy became world number one at just 22 turned from sweet to sour in Tucson last night.

After the joy of beating arch rival Lee Westwood from three down before lunch, McIlroy's bid to dethrone Luke Donald fell flat when he lost the final of the Accenture Match Play Championship 2&1 to American Hunter Mahan.

The Northern Irishman will try again at this week's Honda Classic in Florida - Donald is not playing - but he will be back to facing nearly 140 players there compared to just one.

McIlroy went from playing some sublime golf against Westwood to making mistake after mistake early on against Mahan.

And despite rediscovering his flair on the back nine his opponent - the man whose defeat to Graeme McDowell at Celtic Manor in 2010 handed Europe the Ryder Cup - made sure it was too late and fully earned his second World Golf Championships title.

The US Open champion had already halved two holes in bogeys when Mahan, conqueror of compatriot Mark Wilson in his semi-final, broke the deadlock with a tee shot to two feet on the sixth.

But McIlroy then double-bogeyed the 486-yard next, seeing a chip come back to him after both men had rolled down the slope in front of the green with their approaches, and then had another six on the long eighth after driving into a bunker and going long with his third.

Mahan went three up there despite three-putting for par and when both parred the next he had turned in a level-par 36 to McIlroy's 39.

The gap became four when he missed from nine feet on the 10th after 29-year-old Mahan had almost holed his approach and only did then the player from Holywood spark into life.

He chipped in for eagle at the 11th and had a hat-trick of birdies from the 13th. That was the good news for his fans, but the bad was that Mahan matched two of the birdies and after lipping out for victory from 15 feet on the short 16th another par on the next settled it.

Earlier Westwood had won three of the first four against McIlroy, but opened the door by bogeying the fifth and then saw his former stablemate birdie six of the next eight, all but one of them for wins.

Westwood, now three down, then produced an 18-foot eagle putt after driving the green on the 343-yard 15th, but missed from similar range at the next and conceded defeat after bogeying the 17th.

"It was great," McIlroy told Sky Sports. "It was not the start I wanted. Three down after four is very tough against anybody, but against Lee, who's one of the best in the world, I was just trying to get back to level at some point in the round.

"To turn one up was a bit of a bonus. I made a few birdies and definitely played well to win that one."

He was six under for the 17 holes and Westwood three under.

McIlroy added: "Maybe there's a little extra intensity (at three down), but I think you just have to stay patient and chip away."

Westwood said: "Rory played well. I think we both played well - it was just one of those typical match play games that went a lot with momentum."

He felt he was unlucky on the fifth when he flew the green and considered McIlroy even more fortunate at the long 11th, where the Ulsterman's wild second shot was heading for desert and possibly out of bounds, but hit a cart path and finished on grass near the green. It was halved in birdies.

"That sort of thing can change a match. Go back to level there it's a different kettle of fish. I thought it was big turning point.

"But I'm playing great. I've shot four or five under every day and that's pretty good golf. He can't let a game of match play with all its fickleness get in the way of when you are playing well.

"You build up your confidence and carry that forward."

Winning the final was worth over £882,000, while there was £535,000 for second, £378,000 for third and almost £309,000 for fourth.

The afternoon also went America's way in the third place play-off, Wilson never trailing either as he beat Westwood on the final green.

McIlroy said: "I played a great back nine, which was nice, but just left myself too much to do.

"When I didn't eagle the 15th I knew it was going to be tough because he is very solid from tee to green.

"It wasn't to be. I didn't have my best game with me this week, but I'm happy with how I'm playing and hopefully it won't be long before I'm winning again.

"I've got two more tournaments before the Masters and that's what I'm building up to."

Mahan felt he had beaten "the best player in the world right now for sure" and added: "I felt great with my game coming in, switched putters and was able to run it out.

"We didn't have our best stuff early, but I was able to capitalise on a few miscues."